It has been a practice to obtain a lithographic printing plate by forming a photosensitive resin layer on a substrate having hydrophilic surface, subjecting the thus obtained photosensitive lithographic printing plate to plane exposure (mask exposure) via a lith film and then removing non-image parts by using a developing solution. Owing to the recent digitalization techniques, however, there have been developed computer to plate (CTP) systems whereby a printing plate is directly exposed, without resort to a lith film, by scanning highly directional light such as laser beams on the printing plate in accordance with digitalized image data. Also, there have been developed photosensitive lithographic printing plates appropriate for these systems.
As an example of such photosensitive lithographic printing plates appropriate for laser exposure, a photosensitive lithographic printing plate with the use of a photopolymerizable photosensitive layer can be cited. This is because the sensitivity of a photopolymerizable photosensitive layer can be easily elevated by selecting an appropriate initiator or an appropriate initiation system, compared with other existing photosensitive layers. In such a photopolymerizable photosensitive layer, however, high printing tolerance due to the adhesiveness to the substrate mainly in an image part and favorable printing properties and stain resistance in a non-image part can be hardly achieved at the same time. Thus, various attempts have been made to solve this problem.
As one of these attempts, it is disclosed to employ a diazo compound as an adhesive in a photopolymerizable photosensitive layer (see, for example, JP-A-4-274429). In general, a diazo compound is excellent in the adhesiveness in a negative type photosensitive layer. By combining a diazo compound with a developing solution, improved development properties can be established. However, a diazo compound itself has a low sensitivity and cannot impart a sufficient printing tolerance particularly in a CTP exposure system.
It is also disclosed to form a sol-gel intermediate layer having a polymerizable group (see, for example, JP-A-7-159983). According to this method, favorable printing tolerance can be surely obtained. However, there arise other problems such as insufficient development properties under some development conditions and the occurrence of stains in a non-image part.
Moreover, it is disclosed to use a hydrophilic compound having an ethylenically unsaturated bond (see, for example, JP-A-9-34104). Due to the strongly hydrophilic nature, however, such a compound is frequently affected by dampening water in the step of printing, which brings about another problem that no sufficient printing tolerance can be obtained.